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Where is the Dark Matter?



 
 
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  #1  
Old November 23rd 03, 08:11 PM
Joseph Lazio
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Default Where is the Dark Matter?

"N" == Notorius writes:

N How exactly can anybody be "right" about a theory that hasn't been
N sufficiently proven yet such as "Dark Matter" AKA "Yet Another
N Fudge Factor Theory" akin to Einstein's "Cosmological Constant"?

For the record, dark matter is not a theory. Dark matter is a
hypothesis to account for various discrepancies between theory and
observation.

If one assumes that general relativity is the correct theory to
describe gravity[1], then various observations indicate that there is
far more matter around than we detect via emission of photons. So one
has two options:

1. There is dark matter, matter that is either too faint for us to
detect or in some cases that is hypothesized not to emit photons at
all.

2. General relativity is not an accurate description of gravity.

Most mainstream astronomers and physicists pick #1, for a couple of
reasons. First, GR has passed all of the other experimental tests
thrown and it, and, second, we know examples of things that are
difficult to detect at large distances (e.g., planets) or that do not
emit photons at all (e.g., neutrinos). Some physicists have rejected
#1 and tried to come up with alternate theories for gravity (e.g.,
Modified Newtonian Dynamics, MOND).

[1] On the spatial scales of interest to dark matter, the fact that GR
and quantum mechanics appear difficult to reconcile is not important.

--
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  #2  
Old November 23rd 03, 08:45 PM
George Dishman
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Posts: n/a
Default Where is the Dark Matter?


"Joseph Lazio" wrote in message
...

For the record, dark matter is not a theory. Dark matter is a
hypothesis to account for various discrepancies between theory and
observation.

If one assumes that general relativity is the correct theory to
describe gravity[1], then various observations indicate that there is
far more matter around than we detect via emission of photons. So one
has two options:

1. There is dark matter, matter that is either too faint for us to
detect or in some cases that is hypothesized not to emit photons at
all.

2. General relativity is not an accurate description of gravity.


There has just been a short series of programmes on UK
TV on string theory, based on Brian Greene's "Elegant
Universe", and discussing the hypothesis that gravity
is weaker than other forces because it 'leaks' between
branes. Is there consideration given to the apparent
discrepancy being due to the converse, a leakage of a
gravitational effect into our brane from others?

George


  #3  
Old November 24th 03, 12:46 PM
Joseph Lazio
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Posts: n/a
Default Where is the Dark Matter?

"GD" == George Dishman writes:

GD "Joseph Lazio" wrote in message
GD ...
For the record, dark matter is not a theory. Dark matter is a
hypothesis to account for various discrepancies between theory and
observation.

If one assumes that general relativity is the correct theory to
describe gravity[1], then various observations indicate that there
is far more matter around than we detect via emission of photons.
So one has two options:

1. There is dark matter, matter that is either too faint for us to
detect or in some cases that is hypothesized not to emit photons at
all.

2. General relativity is not an accurate description of gravity.


Of course, there could also be a third option, both of the above are
correct.

GD There has just been a short series of programmes on UK TV on
GD string theory, based on Brian Greene's "Elegant Universe", and
GD discussing the hypothesis that gravity is weaker than other forces
GD because it 'leaks' between branes. Is there consideration given to
GD the apparent discrepancy being due to the converse, a leakage of a
GD gravitational effect into our brane from others?

To the extent that I understand this notion, I don't think so. The
hypothesis is that gravity is weaker than the other forces is based on
the notion that gravity operates on all branes, while the other forces
are confined to our brane. I suppose one might ask whether there is
some "fifth force," similar to gravity that extends across all branes
but is sufficiently weak that we have not yet discovered it. I don't
know enough about string theory to know whether it makes any such
predictions.

--
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  #4  
Old November 28th 03, 06:51 PM
Nicolaas Vroom
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Posts: n/a
Default Where is the Dark Matter?


"Joseph Lazio"
schreef in bericht ...

For the record, dark matter is not a theory. Dark matter is a
hypothesis to account for various discrepancies between theory and
observation.

If one assumes that general relativity is the correct theory to
describe gravity[1], then various observations indicate that there is
far more matter around than we detect via emission of photons. So one
has two options:

1. There is dark matter, matter that is either too faint for us to
detect or in some cases that is hypothesized not to emit photons at
all.

2. General relativity is not an accurate description of gravity.

Most mainstream astronomers and physicists pick #1, for a couple of
reasons. First, GR has passed all of the other experimental tests
thrown and it, and, second, we know examples of things that are
difficult to detect at large distances (e.g., planets) or that do not
emit photons at all (e.g., neutrinos). Some physicists have rejected
#1 and tried to come up with alternate theories for gravity (e.g.,
Modified Newtonian Dynamics, MOND).

[1] On the spatial scales of interest to dark matter, the fact that GR
and quantum mechanics appear difficult to reconcile is not important.


I have great problems in understanding that you can use
the concept of dark matter in order to decide if General Relativity
is right or wrong versus if Newton's Law is right or wrong.

I have no objections if you would have used the advancement
of Mercury's perihelion in order to decide that General Relativity
is a better description of the reality than Newton's law.

The same problem I have also if you can use
the concept of dark matter in order to decide if General Relativity
is right or wrong versus if MOND is right or wrong.

I have three question related to MOND.
1. Does MOND correctly simulate the
advancement of Mercury's perihelion ?
If have a certain doubt because the N in MOND stands for Newton.
2. If I'am correct are there two flavours of MOND:
(standard) MOND and relativity MOND ?
3. Does relativity MOND correctly simulate the advancement
of Mercury's perihelion ?

Are you aware that you can simulate the advancement of
Mercury's perihelion by using Newton's Law and by introducing
a ring of (invisible) darkmatter around the Sun?
I'am not saying that physical that that is allowed
but mathematical it works.

Nicolaas Vroom.
http://users.pandora.be/nicvroom/




  #5  
Old November 28th 03, 08:35 PM
[email protected] \(formerly\)
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Posts: n/a
Default Where is the Dark Matter?

Dear Nicolaas Vroom:

"Nicolaas Vroom" wrote in message
...
....
I have three question related to MOND.
1. Does MOND correctly simulate the
advancement of Mercury's perihelion ?
If have a certain doubt because the N in MOND stands for Newton.


It does not, since it is "arbitrarily" applied to spiral galaxies. It is
not expected to be effective inside a single solar system.

2. If I'am correct are there two flavours of MOND:
(standard) MOND and relativity MOND ?


One flavor that I know of. MOND applies at one scale, and GR at a smaller
scale.

3. Does relativity MOND correctly simulate the advancement
of Mercury's perihelion ?


Not that I know of. Someone may have tried to merge them, but no real
point in it. MOND doesn't describe all that we see in galaxies, and GR
only does if there is Dark Matter present.

David A. Smith


 




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